The present invention relates to electronic packages including a printed wiring board and an electronic component mounted on the board. More particularly, the invention relates to such packages wherein some form of heat transference is utilized to effectively remove heat generated by the component.
As operating speeds and performance of various electronic components such as large scale integrated (LSI) circuit packages increase, these components generate a larger amount of heat. Furthermore, with an increase in packaging density of such components, the temperature of the printed wiring board on which these are positioned also tends to increase. Therefore, the heat control of such components and boards has become increasingly important. One important point in controlling heat transfer is how to dissipate heat from the component to a printed wiring board. Important factors in this heat control include: (1) how to dissipate heat from the component itself; (2) how to dissipate heat from the printed wiring board; and (3) how to mount the component close together on the printed wiring board to effectively conduct heat from the components to the board.
Conventionally, in order to promote heat dissipation from a component such as an LSI circuit module, a metallic part 2 may be located in an LSI component 1, and a heat transfer surface 3a of a printed wiring board 3 joined to part 2 with solder 4 (as shown in FIG. 11). As such, the board 3 in effect functions as a heat sink for the component. The transferred heat is dissipated to the outside (of board 3) through plated through holes (PTHs) 5 formed in the wiring board 3, or may also be transferred to a ground layer 6 and a power supply layer 7 inside the wiring board 3 for eventual external dissipation.
In the aforementioned conventional technique, the heat transfer surface 3a of board 3 is generally a flat metal layer and solder paste is applied to all over the surface 3a including the PTHs 5. The applied solder paste is then reflowed to fix the LSI component to the board. However, during the reflowing process of this known technique, some of the solder paste may flow into the PTHs 5, whereby solder-filled spaces 9 and empty spaces 8 are formed between package 1 and the heat transfer surface 3a. See FIG. 12. In this case, heat can be dissipated from the solder-filled spaces 9 to the outside through board 3 by heat conduction, but heat can be hardly dissipated from the empty spaces 8 because spaces 8 thermally insulate package 1 from surface 3a. For this reason, the temperature of the package increases, which leads to a reduction of the operating speed of the package. Therefore, in this technique, heat generated from package 1 is controlled by ensuring a minimum solder area, focusing on the total amount of heat to be transferred through the solder. However, as seen, the package is not uniformly soldered to surface 3a such that the heat is only locally concentrated and efficient heat dissipation cannot be predicted.
When empty spaces 8 are formed between the package""s bottom surface and the heat transfer surface 3a on printed wiring board 3, flux residues also tend to be left in these spaces. Such flux residues are oxidized and thereby can attack (harm) the package and the reliability of the semiconductor device(s) used therein.
A primary object of the present invention is to join a heat dissipation surface of an electronic component evenly to a heat transfer surface of a printed wiring board to achieve maximized, efficient heat dissipation.
A more particular object of the present invention is to join a heat dissipation surface of an electronic component closely together with a heat transfer surface of a printed wiring board in such a manner so as not to form enclosed empty spaces therebetween.
According to one aspect of the invention, there is provided an electronic package comprising a printed wiring board, an electronic component mounted on the printed wiring board and including a heat dissipating surface, a heat transfer surface located on the printed wiring board and positioned directly beneath the electronic component for receiving heat generated by the electronic component, the heat transfer surface being thermally bonded to the heat dissipating surface and including a plurality of solder-applied areas spaced separately from one another on the heat transfer surface.
According to another aspect of the invention, there is provided an electronic package comprising a printed wiring board including at least one internal conductive layer, an electronic component mounted on the printed wiring board and including a heat dissipating surface, a heat transfer surface located on the printed wiring board and positioned directly beneath the electronic component for receiving heat generated by the electronic component, the heat transfer surface being thermally bonded to the heat dissipating surface and including a plurality of solder-applied areas spaced separately from one another on the heat transfer surface, the heat transfer surface further including a plurality of through holes therein, at least one of the through holes being thermally coupled to the at least one internal conductive layer such that the internal conductive layer will provide a path for heat transfer during package operation.
According to still another aspect of this invention, there is provided an electronic package comprising a printed wiring board, an electronic component mounted on the printed wiring board and including a heat dissipating surface and a heat transfer surface located on the printed wiring board and positioned directly beneath the electronic component for receiving heat generated by the electronic component, the heat transfer surface being thermally bonded to the heat dissipating surface and including a plurality of solder-applied areas arranged in a pattern and spaced separately from one another on the heat transfer surface, some of the solder-applied areas being larger in area than others of said solder-applied areas.
In the present invention, a plurality of separate solder-applied areas are formed on the heat transfer surface to dissipate heat from the electronic component to the printed wiring board. The respective solder-applied areas reliably join the electronic component to the wiring board, so that stable and uniform heat dissipation and reliable solder bonding can be achieved without causing uneven soldering.